It is crucial that the temperature or the like of a substrate processed by, for instance, a substrate processing apparatus, such as a semiconductor wafer (hereafter may be simply referred to as a “wafer”) be measured with a high degree of accuracy in order to accurately control the shapes, the physical characteristics and the like of films, holes and the like formed on the wafer by executing various types of processing such as film formation and etching. Accordingly, various wafer temperature measuring methods have been proposed in the related art, including the use of a resistance thermometer and the use of a fluorescence thermometer that measures the temperature at the rear surface of the base material.
In recent years, research into temperature measuring methods and temperature measuring apparatuses that enable direct measurement of the wafer temperature, which is difficult with the temperature measuring methods in the related art, has gained significant ground (see, for instance, International Publication No. 03/087744). A specific example of a temperature measuring apparatus is now explained in reference to FIGS. 32 and 33. FIG. 32 illustrates the principle of a temperature measuring apparatus in the related art, whereas FIG. 33 is a conceptual diagram of interference waveforms measured with the temperature measuring apparatus.
The temperature measuring apparatus 10 in FIG. 32 is constituted with a low coherence interferometer that may be achieved by adopting the basic principle of, for instance, a Michelson interferometer. The temperature measuring apparatus 10 includes a light source 12 constituted with, for instance, an SLD (super luminescent diode) having low coherence characteristics, a beam splitter 14 that splits the light originating from the light source 12 into two beams, i.e., reference light to be radiated onto a reference mirror 20 and measurement light to be radiated onto a measurement target (e.g., a wafer) 30, the reference mirror 20 drivable along a single direction, with which the optical path length of the reference light can be varied, and a light receiver 16 that receives the reference light reflected at the reference mirror 20 and the measurement light reflected at the measurement target 30 and measures the extent of interference.
In this temperature measuring apparatus 10, the light originating from the light source 12 is split at the beam splitter 14 into two beams, i.e., the reference light and the measurement light. The measurement light is radiated toward the measurement target 30 and is reflected at the two end surfaces (e.g., the front surface and the rear surface) of the measurement target, whereas the reference light is radiated toward the reference mirror 20 and is reflected at the mirror surface. Then, both the reflected measurement light and the reflected reference light reenter the beam splitter 14, and depending upon the optical path length of the reference light, the reflected light beams become superimposed upon each other, thereby inducing interference. The resulting interference wave is detected by the light receiver 16.
Accordingly, the reference mirror 20 is driven along the single direction to alter the optical path length of the reference light for the temperature measurement. Since the coherence length of the light from the light source 12 is small due to the low coherence characteristics of the light source 12, intense interference manifests at a position at which the optical path length of the measurement light and the optical path length of the reference light match and the extent of interference is substantially reduced at other positions under normal circumstances. As the reference mirror 20 is driven along, for instance, the forward/backward direction (the direction indicated by the arrows in FIG. 32) and the optical path length of the reference light is adjusted as described above, the measurement beams reflected from the front surface and the rear surface of the measurement target 30 with different refractive indices (e.g., a refractive index na of the air and a refractive index n of the measurement target 30), and the reflected reference light interfere with each other and, as a result, interference waveforms such as those shown in FIG. 33A are detected.
The distance between the peaks in these interference waveforms is equivalent to the optical path length L, which is indicated by the thickness of the measurement target 30, i.e., the distance between the front surface and the rear surface of the measurement target. With d representing the thickness of the measurement target 30 and n representing the refractive index at the measurement target 30, the optical path length L can be expressed as L=d×n. Since the thickness d and the refractive index n assume varying values as the temperature changes, the optical path length (optical thickness) L at the measurement target 30, too, changes as the temperature changes. Accordingly, based upon the change in the optical path length L at the measurement target 30, the temperature at the measurement target can be measured along the depthwise direction.
As the temperature of the measurement target 30 being heated with a heater or the like changes as shown in FIG. 33, the measurement target 30 expands, as indicated by the one-point chain line. At this time, the refractive index n of the measurement target 30, too, becomes altered and, as a result, the interference waveform position following the temperature change shifts relative to the position prior to the temperature change, which changes the interval between the peak positions, as shown in FIGS. 33A and 33B. The extent to which the peak interval of the interference waveform changes corresponds to the extent of the temperature change. In addition, the distance between the peak positions of the interference waveforms correspond to the distance by which the reference mirror 20 moves. Thus, by accurately measuring the intervals between the peaks in the interference waveforms based upon the distance by which the reference mirror 20 is displaced, the change in the temperature can be measured.
As explained earlier, the optical path length (optical thickness) L of the measurement target 30, which is expressed as thickness d×refractive index n, changes in proportion to the change in the temperature, since the thickness d and the refractive index n both change in proportion to the temperature change. This means that when the thickness d of the measurement target 30 is more significant, the optical path length (optical thickness) L changes by a greater extent relative to the extent of the temperature change and that when the thickness d of the measurement target 30 is less significant, the optical path length (optical thickness) L changes to a lesser extent relative to the extent of the temperature change.
For instance, while the extent of the change in the optical path length L occurring at a silicon wafer with a thickness of 10 mm is 2.7 μm/° C., the optical path length L of a thinner silicon wafer with a thickness of, for instance, 0.75 mm changes by a much smaller extent of 0.2 μm/° C.
When the thickness of the measurement target 30 small, the optical path length L at the measurement target 30 changes by a lesser extent relative to a specific extent of change in the temperature at the measurement target 30, as described above. This means that the length of the peak interval in the interference waveforms corresponding to the individual surfaces of the measurement target 30, which indicates the optical path lengths L, changes to a lesser degree as well, when the thickness d is smaller. In other words, when the measurement target 30 has a smaller thickness d, it is more difficult to accurately measure the extent of change in the peak interval in the interference waveforms corresponding to the individual surfaces of the measurement target 30, presenting a major obstacle to improving the temperature measurement accuracy with which the temperature of the measurement target is measured.